Small quantities of contamination are detrimental to the microchip fabrication process in the manufacturing of semiconductor electronic components. Contaminants may be introduced into the component from many sources such as residues from manufacturing process steps such as lithography, etching, stripping, and chemical mechanical planarization (CMP); particulates either indigenous to and/or resulting from manufacturing processes; inorganic particulates or materials such as native or chemical oxides, metal-containing compounds; or other sources. Contamination in the form of particulates, films, or molecules causes short circuits, open circuits, silicon crystal stacking faults, and other defects. These defects can cause the failure of finished microelectronic circuits, and such failures cause significant yield reductions, which greatly increases manufacturing costs.
Microelectronic circuit fabrication requires many processing steps. Processing is performed under extremely clean conditions and the amount of contamination needed to cause fatal defects in microcircuits is extremely small. For example, an individual particle as small as 0.01 micrometer in size can result in a killer defect in a modern microcircuit. Microcontamination may occur at any time during the many steps needed to complete the microcircuit. Therefore, periodic cleaning of the wafers used for microelectronic circuits is needed to maintain economical yields. Also, tight control of purity and cleanliness of the processing materials is required.
Numerous cleaning methods have been used in the manufacture of semiconductor electronic components. These include immersion in liquid cleaning agents to remove contamination through dissolution and chemical reaction. Such immersion may also serve to reduce the van der Waals adhesive forces and introduce double layer repulsion forces, thereby promoting the release of insoluble particles from substrates. A standard wet cleaning process in common use begins with exposure to a mixture of H2SO4, H2O2, and H2O at 110–130° C., and is followed by immersion in HF or dilute HF at 20–25° C. Next a mixture of NH4OH, H2O2, and H2O at 60–80° C. removes particles, and a mixture of HCl, H2O2, and H2O at 60–80° C. removes metal contamination. Each of these steps is followed by a high purity H2O rinse. This wet cleaning process reaches fundamental barriers at dimensions less than 0.10 micrometer. As the device geometries shrink and gate oxide thickness decreases, sub-micrometer particle removal becomes increasingly difficult.
Stripping/removal of primarily organic photoresist may be performed using dilute aqueous mixtures containing H2SO4 and H2O2. Alternatively, the stripping/removal may be performed using a two-step plasma, or reactive ion etching process, followed by wet chemical cleaning of the residue material. Ozonated H2O has been used for the decomposition of hydrocarbon surface contaminants on silicon wafers.
Brush scrubbing has been used to enhance the liquid immersion process by introducing hydrodynamic shear forces to the contaminated surfaces. A typical application uses a wafer cleaning apparatus comprising two opposed brushes for brushing a vertically disposed wafer in a tank that can contain a process liquid.
The addition of ultrasonic energy can increase the effectiveness of the liquid immersion process. Sound waves vibrating at frequencies greater than 20,000 cycles per second (20 KHz), i.e., beyond the range of human hearing, have been used to transmit high frequency energy into liquid cleaning solutions.
Wet processing methods may become problematic as microelectronic circuit dimensions decrease and as environmental restrictions increase. Among the limitations of wet processing are the progressive contamination of re-circulated liquids, re-deposition from contaminated chemicals, special disposal requirements, environmental damage, special safety procedures during handling, reduced effectiveness in deeply patterned surfaces due to surface tension effects and image collapse (topography sensitivity), dependence of cleaning effectiveness on surface wet-ability to prevent re-adhesion of contaminants, and possible liquid residue causing adhesion of remaining particles. Aqueous cleaning agents that depend upon chemical reaction with surface contaminants may also present compatibility problems with new thin film materials, or with more corrosion-prone metals such as copper. In addition, the International Technology Roadmap for Semiconductors has recommended a 62% reduction in water use by the year 2005 and an 84% reduction by the year 2014 to prevent water shortages. With the continuing trend toward increasing wafer diameters having a larger precision surface area, larger volumes of liquid chemicals will be required in the fabrication process.
In view of these problems, methods for dry (anhydrous) surface cleaning of semiconductor electronic components are being developed. Among these are gas jet cleaning to remove relatively large particles from silicon wafers. However, gas jets can be ineffective for removing particles smaller than about 5 micrometers in diameter because the forces that hold particles on the surface are proportional to the particle size, while the aerodynamic drag forces generated by the flowing gas for removing the particles are proportional to the particle diameter squared. Therefore, the ratio of these forces tends to favor adhesion as the particle size shrinks. In addition, smaller particles are not exposed to strong drag forces in the jet since they normally lie within the surface boundary layer where the gas velocity is low.
Exposure to ozone combined with ultraviolet light can be used to decompose contaminating hydrocarbons from surfaces, but this technique has not been shown to remove inorganic contaminants or particles effectively.
Other alternatives to wet cleaning include the use of jets containing snow or pellet projectiles comprising frozen Ar, N2, H2O or CO2 which are used to “sandblast” contaminated surfaces. In these processes, pressurized gaseous or gas/liquid mixtures are expanded in a nozzle to a pressure near or below atmospheric pressure. The resulting Joule-Thomson cooling forms solid or liquid aerosol particles, which traverse the boundary layer and strike the contaminated surface. This technique requires extremely clean and pure processing materials. Trace molecular contaminants (e.g., hydrocarbons) in the feed gases can condense into solid particulates or droplets upon expansion, causing deposition of new contaminants on the surface. Although useful in providing removal of many surface contaminants, these processes cannot remove all of the important contaminants present on a wafer surface, and have not yet found wide acceptance in the semiconductor industry.
Immersion in supercritical fluids is another alternative to wet cleaning. The effectiveness of supercritical fluids in various cleaning and extraction applications is well established and extensively documented. Supercritical fluids have solvent power much greater than the corresponding gaseous state, and can effectively dissolve and remove unwanted films and molecular contaminants from a precision surface. The contaminants can be separated from the cleaning agent by a reduction in pressure below the critical value, which concentrates the contaminants for disposal and permits recovery and re-use of the cleaning fluid.
Supercritical CO2 in particular has been used as a versatile and cost effective method to overcome the above-mentioned problems in wafer cleaning. Supercritical CO2 effectively cleans parts with increasingly smaller dimensions and lowers water usage, thereby yielding improvements in performance and environmental benefits. Preliminary Cost of Ownership (CoO) studies have shown that supercritical CO2 cleaning is also more cost effective when compared to aqueous cleaning. CO2 in the supercritical state has particularly good solvent properties and has been found to be effective in removing organic impurities. It can be modified with added co-solvents or processing agents to widen the range of contaminants that can be removed, including particles, native or chemical oxides, metallic contaminants, and other inorganic materials. Ultrasonic energy can be introduced into supercritical fluid cleaning reactors to enhance the efficiency of the cleaning process.
Future microcircuits will have smaller feature sizes and greater complexities, and will require more processing steps in their fabrication. Contamination control in the process materials systems and processing environment will become even more critical. In view of these anticipated developments, there is a need for improved wafer cleaning methods to maintain or improve economical yields in the manufacture of these smaller and more complex microelectronic systems. In addition, the advent of smaller feature sizes and greater complexities will require improved fabrication processes steps including etching, thin film deposition, planarization, and photoresist development. Embodiments of the present invention, which are described below and defined by the following claims, address this need by improved processing methods utilizing dense processing fluids with the application of ultrasonic energy.